4. Ocean Floor Spreading

12. Studies of the Mid-Atlantic Ridge in the Atlantic Ocean half-way between North America and Europe have shown that as the fresh rock cools, it records the polarity of Earth's field. By dating the rocks on either side of the ridge, geologists discovered that the polarity of the Earth's field changes over the course of thousands of years. This was an exciting discoverery that not only verified the theory of Continental Drift, but demonstrated that Earth's magnetism isn't constant over millions of years. The magnetic field of Earth actually changes its polarity over time. They are called Polarity Reversals, but should not be confused with the rotation axis of Earth actually changing. The last one of these Polarity Reversals happened about 770,000 years

13. The last one of these Polarity Reversals happened about 770,000 years ago (770 on the above graph). We are currently living during a period that has been called the Brunhes Magnetic Chron when the South Magnetic Pole is in the Northern Hemisphere. During the previous Matumaya Magnetic Chron, the North Magnetic Pole was in the Northern Hemisphere! At the prresent time, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate. The figure above, however, shows that Earth's field often changes its strength significantly in a short time (measured in thousands of years) and often does not vanish everytime. Scientists have a lot to learn about the exact behavior of Earth's complex field. Until then, making predictions about what it may look like in a few thousands years from now is a matter of guessing, not forecasting.

14. There have been about 170 of these reversals during the last 76 million years according to geological evidence. The time between reversals seems to be growing longer, and is currently about 300,000 years or so. The last one of these happened about 770,000 years ago (0.77 on the graph). We are currently living during a period that has been called the Brunhes Magnetic Chron when the South Magnetic Pole is in the Northern Hemisphere. During the previous Matumaya Magnetic Chron, the North Magnetic Pole was in the Northern Hemisphere! Note that, from the polarity figure, at a time 0.94 million years ago (940,000 years ago) the magnetic field reverse itself by going nearly to 'zero' but then after a few thousand years it recovered and began to increase in strength. During the next 150,000 years it rose to a maximum strength and then began to decline. Notice, also, how fast the magnetic field recovers after it reaches 'zero', in some cases much less than 10,000 years.Presently, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate!There have been about 170 of these reversals during the last 76 million years according to geological evidence. The time between reversals seems to be growing longer, and is currently about 300,000 years or so. The last one of these happened about 770,000 years ago (0.77 on the graph). We are currently living during a period that has been called the Brunhes Magnetic Chron when the South Magnetic Pole is in the Northern Hemisphere. During the previous Matumaya Magnetic Chron, the North Magnetic Pole was in the Northern Hemisphere! Note that, from the polarity figure, at a time 0.94 million years ago (940,000 years ago) the magnetic field reverse itself by going nearly to 'zero' but then after a few thousand years it recovered and began to increase in strength. During the next 150,000 years it rose to a maximum strength and then began to decline. Notice, also, how fast the magnetic field recovers after it reaches 'zero', in some cases much less than 10,000 years.Presently, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate! There have been about 170 of these reversals during the last 76 million years according to geological evidence. The time between reversals seems to be growing longer, and is currently about 300,000 years or so. The last one of these happened about 770,000 years ago (0.77 on the graph). We are currently living during a period that has been called the Brunhes Magnetic Chron when the South Magnetic Pole is in the Northern Hemisphere. During the previous Matumaya Magnetic Chron, the North Magnetic Pole was in the Northern Hemisphere! Note that, from the polarity figure, at a time 0.94 million years ago (940,000 years ago) the magnetic field reverse itself by going nearly to 'zero' but then after a few thousand years it recovered and began to increase in strength. During the next 150,000 years it rose to a maximum strength and then began to decline. Notice, also, how fast the magnetic field recovers after it reaches 'zero', in some cases much less than 10,000 years.Presently, Earth's magnetic field is weakening in strength by 5% every 100 years. It may be near zero in another few thousand years at this rate!